AN4007 datasheet(18/33 Pages) STMICROELECTRONICS

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Test board: design and evaluation

AN4007

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Doc ID 022455 Rev 2

Figure 19.

Dynamic step load: 0 to 100% load

1.7

Soft-start

When the converter starts, the output capacitor is discharged and needs some time to reach

the steady-state condition. During this time the power demand from the control loop is the

maximum while the reflected voltage is low. These two conditions could lead to a deep

continuous operating mode of the converter.

When the MOSFET is switched on, it cannot be switched off immediately as the minimum

on-time (TON_MIN) must elapse. Because of the deep continuous working mode of the

converter, during this TON_MIN, an excess of drain current can overstress the component of

the converter as well as the device itself, the output diode, and the transformer. Transformer

saturation is also possible under these conditions.

To avoid all the described negative effects, the VIPER37LE implements an internal soft-start

feature. As the device starts to work, no matter what the control loop requests, the drain

current is allowed to increase from zero to the maximum value gradually.

The drain current limit is incremented in steps, and the values range from 0 to the fixed drain

current limitation value (values that can be adjusted through an external resistor) which is

divided into 16 steps. Each step length is 64 switching cycles. The total length of the soft-

start phase is about 8.5 ms.

Figure 20 shows the soft-start phase of the presented converter

when it is operating at minimum line voltage and maximum load.

AM11358v1

Ch1 (Max): 5.35 V

M: 20.0 µs/div

Ch2 (Max): 3.07 A

Ch1 (Min): 4.58 V

I OUT

VOUT

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